A transponder or tag comprises an integrated circuit and an antenna. The integrated circuit is designed to process a signal captured by the antenna and, in response to the captured signal, to generate a response signal to be transmitted by the antenna. The antenna is normally supported by a substrate and the integrated circuit is attached to the substrate by the so-called flip-chip mounting process.
A known integrated circuit comprises contacts at the surface facing toward the substrate. The contacts are comprised of connection pads connected to bumps, which bumps are electrically connected to the antenna. When producing the transponder, the integrated circuit is attached to the substrate, e.g. by means of a glue layer spread over the substrate. The integrated circuit is then pressed onto the substrate with the glue layer, wherein the surface of the circuit comprising the bumps faces towards the substrate. Because of the pressure, particularly the bumps and the area around the bumps of the integrated circuit, as well as the antenna structure around the area where the bumps press against the substrate, may be subjected to severe stress what eventually leads to deformation of the bumps and the antenna around the bumps. Because of the deformed bumps, the distance between the antenna and the integrated circuit may differ, resulting in different stray capacitances between the antenna and conducting layers of the integrated circuit. Especially when used as an RFID (radio frequency identification) or a UHF (ultra high frequency) tag these different stray capacitances may compromise the performance of the transponder.
Published U.S.-application for patent 2003/0017414 A1 discloses a method of producing a flip chip having solder bumps, an integrated underfill, and a separate flux coating. Initially, a semiconductor device having connection pads on a surface thereof and an underfill material applied to the surface is processed so that the underfill material forms apertures positioned precisely at the connection pads. The main component of the underfill material is a thermoplastic resin. The apertures are made utilizing a photoablation process, because a photoablation process needs only a minimum amount of heating. Then, the apertures are filled with a solder material to form solder bumps electrically connected to the pads. The solder bumps extend slightly above the underfill material.
When mounting this flip chip on a substrate by pressing the flip chip against the substrate and heating up the solder bumps, then the thermoplastic resin of the underfill softens or even melts and hence likely is deformed what results in different distances between the surface of different flip chips and their substrates. This results in different stray capacitances between the conducting layers of the substrate and conducting layers of the flip chip what in turn may lead to malfunction of the transponder.